1. Field of the Invention
The present invention relates to a thin-film magnetic head incorporating a magnetoresistive element utilizing a tunnel magnetoresistive effect, and to a head gimbal assembly, a head arm assembly, and a magnetic disk drive each of which incorporates the thin-film magnetic head.
2. Description of the Related Art
Performance improvements in thin-film magnetic heads have been sought as areal recording density of magnetic disk drives has increased. A widely used type of thin-film magnetic head is a composite thin-film magnetic head that has a structure in which a write (recording) head having an induction-type electromagnetic transducer for writing and a read (reproducing) head having a magnetoresistive (MR) element for reading are stacked on a substrate.
MR elements include: anisotropic magnetoresistive (AMR) elements utilizing an anisotropic magnetoresistive effect; giant magnetoresistive (GMR) elements utilizing a giant magnetoresistive effect; and tunnel magnetoresistive (TMR) elements utilizing a tunnel magnetoresistive effect.
It is required that the characteristics of a read head include high sensitivity and high output capability. GMR heads incorporating spin-valve GMR elements have been mass-produced as read heads that satisfy such requirements. Recently, developments in read heads using TMR elements have been sought to conform to further improvements in areal recording density.
Typically, a TMR element incorporates: a tunnel barrier layer having two surfaces facing toward opposite directions; a free layer disposed adjacent to one of the surfaces of the tunnel barrier layer; a pinned layer disposed adjacent to the other of the surfaces of the tunnel barrier layer; and an antiferromagnetic layer disposed adjacent to one of the surfaces of the pinned layer farther from the tunnel barrier layer. The tunnel barrier layer is a nonmagnetic layer that allows electrons to pass therethrough while the electrons maintain spins by means of the tunnel effect. The free layer is a layer in which the direction of magnetization changes in response to a signal magnetic field. The pinned layer is a ferromagnetic layer in which the direction of magnetization is fixed. The antiferromagnetic layer is a layer that fixes the direction of magnetization in the pinned layer by means of exchange coupling with the pinned layer.
In the TMR element the direction of magnetization of the free layer changes in response to the signal field sent from a recording medium. The relative angle between the direction of magnetization of the free layer and the direction of magnetization of the pinned layer thereby changes. If the relative angle changes, the probability of electrons passing through the tunnel barrier layer while maintaining spins changes. As a result, the resistance of the TMR element changes. Data stored on the recording medium is read by detecting the changes in resistance of the TMR element. It is expected for the TMR element to obtain a magnetoresistance change ratio (hereinafter referred to as MR ratio), that is, a ratio of magnetoresistance change with respect to the resistance, greater than an MR ratio obtained with a spin-valve GMR element.
Typically, the tunnel barrier layer is an insulating layer made of an insulating material such as aluminum oxide or magnesium oxide.
Japanese Published Patent Application 6-97531 discloses a magnetoresistive element having a structure in which a semiconductor layer is sandwiched between two magnetic layers. In the magnetoresistive element a Schottky barrier between the semiconductor layer and each of the magnetic layers is used as a tunnel barrier. This publication discloses that it is possible that the semiconductor layer has a thickness greater than the thickness of a tunnel barrier layer of a TMR element in which the tunnel barrier layer is made of an insulating layer.
Japanese Published Patent Application 2001-94172 discloses a spin conduction element having a structure in which a tunnel barrier layer made of an insulator or a semiconductor is sandwiched between a ferromagnetic electrode and another electrode made of a paramagnetic metal, a ferromagnetic metal or a semiconductor. This publication lists materials of the tunnel barrier layer that make it possible to obtain a tunnel current having a great spin polarization.
Japanese Published Patent Application 2002-84014 discloses a magnetoresistive element having a structure in which a second layer made of a nonmagnetic insulating film is sandwiched between a first layer made of a ferromagnetic metal thin film and a third layer made of a ferromagnetic metal thin film. This magnetoresistive element is designed such that the spin polarization of the second layer is one tenth or smaller of the spin polarization of each of the first and third layers. A ZnOx (x=0.95 to 1.05) thin film as an insulating film is used as the second layer. This publication discloses that if such a second layer is used, it is possible to improve the MR ratio of the magnetoresistive element and to improve performance of the magnetoresistive element such as detection sensitivity.
Japanese Published Patent Application 2006-86476 discloses a magnetic write element comprising: a free layer in which the direction of magnetization changes due to the function of spin-polarized electrons; a pinned layer in which the direction of magnetization is fixed; and an intermediate layer made of a nonmagnetic material and provided between the pinned layer and the free layer. This publication lists a nonmagnetic metal material, a nonmagnetic semiconductor material and an insulating material as the material of the intermediate layer. The magnetic write element is designed so that the direction of magnetization of the free layer is changed by feeding spin-polarized electrons to the free layer.
When a TMR element is used for a read head, a reduction in resistance of the TMR element is required. The reason will now be described. For a magnetic disk drive, improvements in recording density and data transfer rate are required. An excellent high frequency response is required for the read head, accordingly. However, if the resistance of the TMR element is high, stray capacitance produced in the TMR element and in circuits connected thereto is increased, and the high frequency response of the read head is thereby degraded. A reduction in resistance of the TMR element is therefore required.
Typically, it is effective to reduce the thickness of the tunnel barrier layer for reducing the resistance of the TMR element. However, if the tunnel barrier layer is made too thin, magnetic coupling is established between the free layer and the pinned layer, which results in degradation of characteristics of the TMR element such as an increase in noise and a reduction in MR ratio.
Furthermore, if the TMR element is increased in size, the resistance thereof is reduced. However, an increase in size of the TMR element is against the demand for improving the recording density.
A variety of types of magnetoresistive elements having features in the layer corresponding to the tunnel barrier layer of the TMR element have been proposed, as disclosed in Japanese Published Patent Application 6-97531, Japanese Published Patent Application 2001-94172, Japanese Published Patent Application 2002-84014, and Japanese Published Patent Application 2006-86476. However, a reduction in resistance of the magnetoresistive element is considered in none of these publications.